This invention relates generally to measuring frequency, and more particularly to methods and systems for measuring a rate of change of frequency.
Mechanical frequency limits are sometimes established to protect an electrical generator prime mover, typically a turbine. For example, it may be generally undesirable to operate a turbine above or below predetermined mechanical resonance limits and/or frequencies. Accordingly, it may be important to monitor an operating frequency of the turbine. An electrical signal frequency, indicative of turbine mechanical frequency, may therefore be monitored to determine whether the frequency is within predetermined electrical frequency limits, and thereby facilitate operating the turbine within the predetermined band of mechanical frequencies. Moreover, before interconnecting loads supplied by different generators, it may be desirable that both generators are operating at the same electrical frequency to prevent voltage surges and voltage transients, which may damage the loads should interconnection of non-synchronous generators be attempted.
At least some known electrical power frequency and frequency rate of change measurement techniques measure the time between zero crossings of the voltage waveform, and generate frequency and/or frequency rate of change values from the zero crossing measurements. For example, at least some known measurement techniques include estimating frequency from the zero crossing measurements, and estimating frequency rate of change by subtracting pairs of frequency measurements. However, at least some of such known measuring techniques may be sensitive to noise on the power lines because of large frequency changes naturally occurring in the utility network, voltage spikes, and/or voltage glitches, which may be detected as zero crossings of the voltage waveform thereby leading to erroneous frequency determinations. Moreover, non-fundamental components of the electrical power system may produce signals detected as zero crossings, which may also produce erroneous frequency results.
Improper and/or erroneous frequency determinations may lead to costly premature system shutdown if the system frequency does not actually exceed the predetermined frequency limits and the frequency monitor indicates it does exceed the limits. Moreover, if the system frequency does actually exceed the predetermined limits and the frequency monitor indicates it does not exceed the limits, improper and/or erroneous frequency determinations may result in damage to a prime mover and/or other electrical power system components. Furthermore, non-synchronous generators may be interconnected, resulting in problems described above, for example if the frequency monitor erroneously indicates that the frequencies of the generators are the same. At least some known frequency measurement techniques that use zero crossing measurements may also be less accurate than desirable, for example if the measured signal is not a perfect sinusoid. Moreover, at least some known electrical power systems include distributed generators, which may sometimes require anti-islanding control. However, at least some known frequency measurement techniques that use zero crossing measurements may include a non-detection zone (NDZ), and therefore may not be able to prevent unintentional islanding under power match conditions.